SPN 1112 FMI 7 — Engine Compression Brake Mechanical System Not Responding
FMI 7 is a functional 'not responding' fault. Prove electrical power, interlocks, and oil pressure before mechanical work.
Fault Overview
FMI 7 indicates the mechanical or functional response of the compression brake does not match the command. The ECM may monitor engine speed change, manifold pressure response, or a feedback signal and determine the brake is not producing expected retardation.
Engine & Platform Coverage
Symptoms
- •Jake brake not responding
- •Compression brake no effect
- •Engine brake performance fault
- •Brake commanded but no retardation
Possible Causes
- 1.Failed or stuck solenoid
- 2.Low oil pressure or restricted oil supply to the brake actuators
- 3.Incorrect lash or mechanical adjustment
- 4.Wiring that passes electrical checks but has high resistance under load
- 5.Software enable conditions not met (clutch, ABS, cruise interlocks)
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
4 steps locked behind Technician Mode- 1Step 1 — Verify Circuit Voltage and Current Under Command: Confirm voltage and current at the solenoid under commanded load are within OEM specification. A circuit that passes static checks but has high resistance under load will cause FMI 7.
- 2Step 2 — Confirm All Interlocks Allow Brake Operation: Verify clutch, neutral, ABS, and cruise interlocks are all in the required state for brake operation. A single inhibited interlock will prevent the brake from engaging even when the circuit is good.
- 3Step 3 — Monitor Engine Speed or Load Response: Monitor engine speed deceleration or boost/MAP response during commanded braking if available. Confirm the brake is producing measurable retardation.
- 4Step 4 — Check Oil Pressure at Brake Operating Conditions: Measure oil pressure at idle and under the RPM and load conditions where the brake is used. Oil pressure must meet OEM minimum for brake operation. Low oil pressure is a classic cause of weak or inoperative Jake brakes.
- 5Step 5 — Inspect Mechanical Condition and Lash: Inspect mechanical condition and lash adjustment if accessible and if all previous steps pass. Follow OEM functional test procedure if available.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Replace failed solenoid or actuator.
- 2Correct oil supply restriction.
- 3Adjust lash per OEM specification.
- 4Repair high-resistance connections.
- 5Address interlock faults.
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Start Diagnosing This Fault Now
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Related Resources
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Summary
FMI 7 is a functional 'not responding' fault. Prove electrical power, interlocks, and oil pressure before mechanical work.